Trying to make a minimal version

[brianjenkins94]

But I can't make sense of this:

https://github.com/eta-dev/eta/blob/master/src/compile-string.ts#L25-L41

[brianjenkins94]

// TODO: Rewrite this using the regex sticky flag
function parse(template) {
	template = template
		.replace(/(?<!:)\/\/.*?$/gmu, "") // comments
		.replace(/\\/gu, "\\\\") // backslashes
		.replace(/"/gu, "\\\"") // double quotes
		.replace(/`/gu, "\\`") // backticks
		.replace(/\$\{/gu, "\\${"); // substitutions

	const buffer = [];

	const openTagRegex = /(.*?)<%\s*(=|-)?\s*/gsu;
	const closeTagRegex = /'|"|`|\/\*|(\s*%>)/gu;

	let index = 0;
	let match;

	while (match = openTagRegex.exec(template)) {
		index = match[0].length + match.index;
		const prefix = match[2] ?? "";

		buffer.push(match[1]);

		closeTagRegex.lastIndex = index;

		let closeTag;

		while (closeTag = closeTagRegex.exec(template)) {
			if (closeTag[1] !== undefined) {
				const value = template.substring(index, closeTag.index);

				index = closeTagRegex.lastIndex;
				openTagRegex.lastIndex = index;

				buffer.push({
					"type": {
						"": "execute",
						"-": "raw",
						"=": "interpolate"
					}[prefix],
					"value": value
				});

				break;
			}
		}
	}

	if (index < template.length) {
		buffer.push(template.substring(index));
	}

	return buffer;
}

function compile(nodes) {
	const buffer = ["const buffer = [];"];

	for (const node of nodes) {
		if (typeof node === "string") {
			buffer.push("buffer.push(`" + node + "`);");
		} else {
			let { type, value } = node;

			switch (type) {
				case "interpolate":
					if (/[&<>"']/gu.test(value)) {
						value = value.replace(/[&<>"']/gu, function(character) {
							return {
								"&": "&amp;",
								"<": "&lt;",
								">": "&gt;",
								"\"": "&quot;",
								"'": "&#39;"
							}[character];
						});
					}
				case "raw":
					buffer.push("buffer.push(`" + value + "`);");
					break;
				case "execute":
					buffer.push(value);
					break;
				default:
			}
		}
	}

	buffer.push("return buffer.join(\"\\n\");");

	return buffer.join("\n");
}

function render(template, data = {}) {
	// eslint-disable-next-line no-new-func, @typescript-eslint/no-implied-eval
	return new Function(...Object.keys(data), compile(parse(template)))(...Object.values(data));
}

console.log(render("<%= foo %>\n<%= bar %>\n<%= baz %>\n<%= 2 + 2 %>\n<%- \"<br />\" %>", {
	"foo": "bar",
	"bar": "baz",
	"baz": "foo"
}));